Blowflies have long created headaches for Australian sheep farmers because they lay their eggs in faecal matter around the sheep's tail, causing blowfly 'strike'.

The pesticide diazinon was introduced into Australia in the 1950s as a sheep 'dip' to combat the problem. But within six years blowflies developed a high level of resistance to it.

Researchers from CSIRO Entomology in Canberra and collaborators from New Zealand and the UK wanted to see if the blowflies had a pre-adaptation resistance to the insecticide.

This would explain why they developed resistance so rapidly.

To do this CSIRO's Dr Carol Hartley and colleagues gathered 150 samples, kept at the Australian National Insect Collection, of 70-year-old blowfly legs from two Australian sheep blowflies Lucilia cuprina and its close relative L. sericata.

They then crushed them and extracted their DNA.

The researchers compared the state of blowflies' resistance genes before and after the introduction of the pesticide.

"We didn't find any diazinon resistance in the old fly legs, whereas it occurs in the present day species. That tells us that in the case of the diazinon pesticide there was no pre-adaptation."

But when the researchers conducted the same experiment with another pesticide malathion, they found resistance genes both in the old blowflies and the modern blowflies, showing there was pre-existing resistance.

"This is evidence for pre-adaptation as a mechanism of evolution of insecticide resistance," says Hartley.

She says it is this pre-existing resistance that let resistance to organophosphate insecticides take off so rapidly. The specific diazinon resistance gene developed later.

"We used the DNA from the very old flies to show that one form of the alteration already existed in the flies from 70 years ago, long before they had ever been exposed to the organophosphate chemicals," Hartley says.

"It was this that allowed the flies to rapidly develop resistance to the organophosphate chemicals in general, after which the better version of the mutation took over in the flies."

She says while the research mainly tells us about the mechanism of evolution and how it occurred in these particular blowfly species, it also gives a clue to why rapid evolution of resistance can occur.

This might help researchers predict potential insecticide resistance in the future, she says.